1. Field of the Invention
This invention relates to electromagnetic wave components whose resonant frequency band can be tuned, especially components for microwaves and radio frequencies.
2. Description of the Related Art
Electromagnetic wave components are widely used in research and industry, especially for communications in the microwave region. Their physical dimensions determine which frequencies and modes will propagate through them, and filters can be constructed to eliminate unwanted frequencies. However, it is often necessary to tune an electromagnetic wave component, such as a cavity, filter, or coaxial resonator, so that it responds precisely to a particular frequency of interest. For example, the resonant frequency or frequencies of a component will change with temperature, since the component will expand or contract in accordance with its thermal coefficient of expansion, thereby varying the frequencies supported by the cavity. A component's temperature can significantly increase due to thermal effects resulting from its operation.
An electromagnetic wave component is frequently tuned with screws that penetrate through its walls into its interior where the screws interact with propagating electromagnetic waves, especially with their electric field component, to vary the allowed frequencies of propagation. This technique is illustrated by T. Nishikawa, K. Wakino, H. Wada and Y. Ishikawa in "800 MHZ Band Dielectric Channel Dropping Filter Using TM.sub.110 Triple Mode Resonance," 1985 IEEE MTT-S International Microwave Symposium Digest, Jun. 4-6, 1985, pp. 289-292, St. Louis, Mo. Tuning screws are generally metal, since dielectrics tend to dissipate microwave energy. They are sometimes fitted with an object such as a disc on their end, in order to increase the effective surface area.
"Orthogonal tuning screws" are screws positioned in line with the electric field components of the electromagnetic wave component's two orthogonal modes. Typically, each of the two orthogonal modes will have its own set of screws, so that the modes can be tuned independently of each other. Other screws not perpendicular to the orthogonal tuning screws are often used to vary the degree of coupling between the two orthogonal modes. FIG. 1 shows a dispersion relation 10 that represents the transmission through an electromagnetic wave component in the absence of any screws. The resonant frequency band is shifted when screws penetrate into the component's interior, as represented by dispersion relation 12.
The use of screws has a number of shortcomings, however. For one, screws are limited in the extent to which they can tune a system because of their small surface area. Also, disruptions of the electromagnetic field at metal-to-metal contact points (e.g. where the screw enters the cavity) can lead to the passive intermodulation (PIM) problem in high power devices. For this reason, the mechanical tolerances of the screws and their holes must be kept tight, and an additional filter must be frequently added to such a system to eliminate unwanted frequencies. Most importantly, the use of screws leads to turbulence in the electromagnetic waves, resulting in resistive losses. In general, turbulence can be expected in any system in which the electromagnetic waves encounter edges or protrusions.
Tuning blocks, which are metallic or dielectric "buttons" secured with adhesive onto an interior wall of an electromagnetic wave component, suffer in general from the same problems as screws. To circumvent the problems associated with screws or buttons, pliers are sometimes used to deform an electromagnetic wave component, as for example in the procedure known as "dent tuning." That is, when the walls of the component are deformed, the modes it supports are altered. However, it is difficult to reverse the damage done to a structure that has been dented, and the degree of tunability offered by this procedure is highly dependent upon the user's experience.
To mitigate drifts in their resonant frequency or frequencies due to changes in temperature, electromagnetic wave components are often constructed from alloys such as nickel-steel which are temperature-stable but expensive and heavy. Even so, these alloys do not offer a complete solution to the problem of frequency drift, since the resonant frequency of such devices still drifts by as much as several tenths of a percent over typical operating conditions. A temperature compensating waveguide resonator is described in U.S. Pat. No. 4,677,403 to Kich which partially compensates for drifts in the resonant frequency arising from thermal expansion or contraction of the resonator, but it does not allow for active tuning.